1. Field of the Invention
The present invention relates to a DC-DC converter to be provided in apparatus such as a switching power-supply switching device.
2. Description of the Related Art
FIG. 13 shows an example of a DC-DC converter. As shown in the drawing, the DC-DC converter is an isolated forward converter having a transformer 1. The transformer 1 has a primary coil N1, a secondary coil N2, and voltage-detecting coil N3. A primary side circuit 3 having a switching device Q and a capacitor 2 is connected to the primary coil N1. An input power supply 4 is connected to the primary side circuit 3.
Also, a secondary side circuit 5 is connected to the aforementioned secondary coil N2, and an output side of the secondary side circuit 5 is connected to a load 6. In addition, a voltage-detecting circuit 8 is connected to the voltage-detecting coil N3. An output side of the voltage-detecting circuit 8 is connected to a control circuit 10. An output of the control circuit 10 is connected to the switching device Q of the primary side circuit 3.
The primary side circuit 3 has a configuration in which power is sent from the input power supply 4 to the secondary side circuit 5 via the transformer 1. The secondary side circuit 5 rectifies and smoothes voltage from the secondary coil N2, and a DC voltage V.sub.out rectified and smoothed thereby is outputted to the load 6.
Also, the voltage-detecting circuit 8 has a configuration that rectifies and smoothes voltage from the voltage-detecting coil N3. A voltage corresponding to a voltage from the aforementioned secondary coil N2 is generated, and the voltage from the secondary coil N2 is processed as a voltage corresponding to an output voltage V.sub.out of the aforementioned secondary side circuit 5. Accordingly, the voltage-detecting circuit 8 has a configuration that outputs the aforementioned voltage rectified and smoothed into the control circuit 10 as the output voltage V.sub.out of the secondary side circuit 5.
In addition, according to a detected voltage applied by the voltage-detecting circuit 8, the control circuit 10 has a configuration that applies a pulse control signal to the switching device Q, the pulse control signal functioning to stabilize the output voltage V.sub.out of the secondary side circuit 5, as shown in FIG. 12. The pulse control signal controls on/off operations of the switching device Q, and the switching device Q performs on/off operations according to the pulse control signal.
As described above, in the DC-DC converter shown in FIG. 13, a voltage corresponding to the output voltage V.sub.out of the secondary side circuit 5 is generated in the voltage-detecting coil N3. This allows the voltage-detecting circuit 8 to use the voltage in the voltage-detecting coil N3 and to indirectly detect and output the output voltage V.sub.out of the secondary side circuit 5. By circuit operations of the control circuit 10 according to the detected voltage, on/off operations of the switching device Q are controlled, and the output voltage V.sub.out of the secondary side circuit 5 is stabilized.
In the foregoing DC-DC converter in which the output voltage V.sub.out of the secondary side circuit 5 is indirectly detected, problems occur because output voltage V.sub.out varies according to load variation and this variation is not reflected in the voltage detected by voltage detecting circuit 8.
That is, when load variation occurs, currents I.sub.out to be outputted to the load 6 vary according to the load variation. This current variation causes the output voltage V.sub.out to also vary. On the other hand, no variation occurs in a voltage applied from the voltage-detecting circuit 8 to the control circuit 10 and the control circuit 10 does not compensate for the variations in the output voltage V.sub.out attributable to the variations in load 6.
In other words, in the circuit configuration as shown in FIG. 13, the relationship between the current I.sub.out and the output voltage V.sub.out, as shown in FIG. 11, is produced, and the output voltage V.sub.out varies according to variation of the current I.sub.out caused by variations in the load 6. This load-variation-attributed variation of the output voltage V.sub.out cannot be detected by the voltage-detecting coil N3. Therefore, in the control circuit 10, circuit operations for compensating the variation of the output voltage V.sub.out do not occur. This produces problems in that the voltage V.sub.out varies according to the load variation, and therefore, the output voltage V.sub.out is not stabilized satisfactorily.
The following problems also arise in a DC-DC converter of a type in FIG. 13, in which the output voltage V.sub.out of the secondary side circuit 5 is indirectly detected. When environmental variations occur for the DC-DC converter, characteristics of components of the individual circuits vary according to variation of the temperature therein, resulting in variation of the output voltage V.sub.out of the secondary side circuit 5. This causes problems in that variation of the output voltage V.sub.out due to temperature-variation cannot be prevented.
These problems are caused by the following reasons. As in the case where the aforementioned load variation occurs, when the output voltage V.sub.out varies according to environmental temperature variations, the variation of the output voltage V.sub.out cannot be detected using the voltage-detecting coil N3. In addition, the detected voltage outputted from the voltage-detecting circuit 8 to the control circuit 10 also varies according to the aforementioned environmental temperature variation without regard to the output voltage V.sub.out. Since on/off operations of the switching device Q are controlled by circuit operations of the control circuit 10 according to the detected voltage, problems arises in that the output voltage V.sub.out of the secondary side circuit 5 is very unstable because of the temperature variation.